ip6mr: ip6mr_cache_report() changes

ip6mr_cache_report() first argument can be marked const, and we change
the caller convention about which lock needs to be held.

Instead of read_lock(&mrt_lock), we can use rcu_read_lock().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>